A little more experimentation produces some oddities.

If I leave my tuners A~G as is then I get successful channel scans on all of the dSCR-54's UB's other than 9(1280), 11(1380) and 14(1480). I was surprised by 9(1280) as this frequency appears in other produces in unicables.xml. This implied that UB=3 and UB=16~26 (as per the table on the dSCR-54's cover). However setting the tuners to a contiguous range in the 16-26 block and most fail with tuning errors/no duration recordings.

I've gone back to the channel selections I referenced above and everything is rock solid again